The present invention relates generally to a shift arrangement for power transfer units and, more particularly, to a synchronized shift arrangement for a planetary gear assembly which may be incorporated into the transfer case of a four-wheel drive vehicle.
Modernly, many light-duty and sport-utility vehicles are equipped with a power transfer unit, such as a four-wheel drive transfer case, for providing a four-wheel drive mode of operation. To accommodate differing road surfaces and conditions, many four-wheel drive transfer cases are equipped with a gear reduction unit for providing a high-range (i.e., direct drive) and a low-range (i.e., reduced ratio drive) in conjunction with the four-wheel drive mode. Most commonly, the gear reduction units used in four-wheel drive transfer cases include either a layshaft arrangement or a planetary gear assembly. Reference may be made to U.S. Pat. No. 4,770,280 for disclosure of an exemplary four-wheel drive transfer case equipped with a dual-speed planetary gear assembly.
Typically, a four-wheel drive transfer case equipped with conventional gear reduction unit can only be shifted between the four-wheel low-range mode and the four-wheel high-range mode when the motor vehicle is in a substantially non-motive condition. During "off-road" use however, the need to stop the vehicle prior to shifting into the four-wheel high-range mode tends to be inconvenient, particularly upon encountering terrain wherein continuation of the vehicle's rolling momentum would provide a smoother transition from four-wheel low-range operation to four-wheel high-range operation. U.S. Pat. No. 5,054,335 to Andrews discloses a four-wheel drive transfer case having a layshaft-type gear reduction unit that can be shifted "on-the-fly" from the four-wheel low-range mode to the four-wheel high-range mode. While such a design has proven to be an advance over conventional layshaft constructions, the teachings are inapplicable to modern four-wheel drive transfer cases equipped with planetary-type gear reduction systems. As such, the need exists for the development of a shift arrangement for shifting the planetary-type gear reduction unit of a four-wheel drive transfer case from the low-range operating mode to the high-range operating mode without stopping the vehicle.